The rotor of an electrical machine may comprise a magnet secured to a shaft. The tensile strength of most magnets is relatively weak. As a result, the magnet may fracture when rotating at high speed. Consequently, for high-speed applications, a reinforcing sleeve is often provided around the magnet.
The sleeve is ideally prestressed such that the magnet is maintained under compression. For a sleeve having a relatively high co-efficient of thermal expansion (e.g. steel), prestress may be achieved by heating the sleeve prior to mounting about the magnet. During subsequent cooling, the sleeve contracts and compresses the magnet. Unfortunately, this method of assembly cannot be used for a sleeve having a relatively low or negative co-efficient of thermal expansion (e.g. carbon-fiber or aramid composite). Additionally, relatively tight tolerances in the outer diameter of the magnet and the inner diameter of the sleeve are often required in order that the difference in diameters can be accommodated by the thermal expansion of the sleeve.